Electroluminescent cell and method of making same



ELECTROLUMINESCENT CELL AND METHOD OF MAKING SAME] Filed Jan. 8, 1957 6 u Q? N L. m \f s N m M 2 m? a II C -V Z 4 3 m 6 5 ATTORNE Y 5 llLEC'I'ROLUMINESCENT CELL AND METHOD or MAKING SAME Elemer Nagy and Janos Szabo, Budapest, Hungary, as-

signers to Egyesult Izzolampa Es Villamossagi Reszvenytarsasag, Budapest, Hungary Filed Jan. 8, 1957, Ser. No. 633,121

11 Claims. (Cl. 313-108) This invention relates to electroluminescent cells and methods of making them. r

With known electroluminescent cells light is obtained by the application of voltage across a phosphor or by placing the phosphor in an electric field, the cells being most efiective when used on alternating, pulsating or otherwise varying currents or fields. The phosphor is provided between two electrodes, at least one of which is a light transmitting or transparent layer of conducting material applied to an insulating substance such as e.g. glass. For the same purpose other conducting materials, such as the so called conductive glass, have also been proposed. The two insulating sheets together with the two conducting electrodes and the phosphor sandwiched in between the two electrodes form the electroluminescent cell.

As Destriau showed the quantity of light radiated by, this kind of electroluminescent cells operated with A.C. of a given frequency rapidly increases with the electric field set up on the electroluminescent substance. It results therefrom that in order to attain high efficiency electroluminescent cells must be operated with great field strength. Since these cells are simply connected to the usual mains, to to about 220 volts, a great field cannot be attained but with very thin electroluminescent layem.

The required field is practically always higher than the dielectric strength of the air (about 21 kvn/cm. with an A.C.). Therefore the slightest remains of air must be removed from the cell. For this purpose the phosphor consisting of small pulverized particles is mixed'with an organic binding material and then spread out into a thin, film-like layer and the electrodes appliedto both sides of said layer. V

Since the phosphor as usually applied is of the zinc sulphide type with a dielectric constant of at least 12 and even greater, whereby the dielectric constant of the binding material filling the interstices between said particles of the phosphor is generally lower, the voltageapplied across the phosphor causes a greater stressin the binding material and a lower stress in the phosphor in dependence of said dielectric constants. Therefore, in order to attain the necessary field, either the usual mains voltage was insufiicient, or the distance between the electrodes had to be shortened to such a degree that difficulties arose in the manufacture of an electroluminescent layer of the needed thickness. Thereby the danger of breakdowns between the electrodes was increased.

It is an object of the invention to provide an electro luminescent, cell in which the foregoing defects are overcome.

Another object of the invention is to provide an electroluminescent cell adapted to be operated on the usual mains voltage, with economic lumen/watt values.

Still another object of the invention is to increase the light efiiciency of electroluminescent cells.

The novel electroluminescent cell according to our invention comprises two electrodes, at least one of which made of a light transmitting material, a layer between niobates or mixtures of said substances with each-other ICE said electrodes of a field-responsive phosphor embedded in a mixture of a bindingmaterial and an inorganic filling substance, said filling substance having a dielectric constant of at least 20 and a leakage constant (tangent 6) not exceeding 0.2.

It has been found that in order to overcome the above those of the electroluminescent powdered material fills the interstices between the granules of the electroluminescent material with a very high space factor. Thus the quantity of the organic binding material will be consider-.

ably less than required hitherto, and hence the electric field will be set up on the electroluminescent powder by a substantially greater part of the total voltage impressed upon the electrodes. Thus a quite smaller part of said voltage will drop on the binding material. Consequently only a small increase of the field will occur upon the binding material and since the quantity of this material has been decreased, in the end the voltage and thereby the field upon the granules of the phosphor will be increased. Owing to its high dielectric constant the potential difference across the filling material is practically negligible. I

We prefer, however, to add an inorganic filling substance to the phosphor mixed with a binding material, the dielectric constant of said inorganic substance being at least 20, but preferably substantially higher, at least about 50 and having a leakage constant (tangent 6) of at most 0.2.

The filling substance of this high dielectric constant is a finely powdered material the granules of which having a size smaller than that of the phosphor. Since the size of the granules of the phosphor is generally over 5 microns, the size of the granules of the filling substance is smaller than 5 microns and is preferably of about 1-2 microns or even less.

As an inorganic substance any one may be used which complies with the above said requirements and does not afiect disadvantageously the operation of the cell. It has been found that titanium dioxide is well adapted to this purpose as well as generally oxides of the elements of group IV of the'periodic system having a dielectric con or niobates, especially alkaline earth titanates, alkaline earth zirconates, alkaline earth hafnates, alkaline earth or with the respective oxides. Good results have been obtained for instance with barium titanate which has a high dielectric constant.

As a binding material we preferably use derivates of cellulose, such as nitrocellulose or the like, but various other materials may be used as described in our co-pend ing application Ser. No. 633,122, now abandoned, con-' cerning an Electroluminescent Cell and Method of ing Same. These materials contain in addition to col-' lodion or artificial resins produced by condensation an Good results have been obtained by using filling material. The quantity of the; filling material to beused;

depends also upon the grain size of the substance.

As an electroluminescentmaterial any known phosphor 1 be used. We prefer to use phosphors of the ZnS may p should be about. l0% but at the most 200% by weight of the phosphor.

V 'viewing his myenuong hew v rg w utilize'plates I made f icongluc ting niaterial iich'asj'condueting glas'e m which case thecondncting'flayers ejana 4 mgyitj igi have size of abont 8 micrbnsffor instance "zir'itisulp V 7 smt ma e y m xed-w th a pow r d' o am si t tan V 105 about 25 :microns.u

' -ri sin at ria is" sa e? w ll ss "The foregoing 'and other objects, features and advert tages of the invention will be understood from the following detailed description referring to the accompany-V ing drawing showing one embodiment of the invention and its' 's'cop'e will be pointedout'in the appended claims;

" The figureiof the drawing shows a perspec'tive"' vier-i;

partly "cutaway, of-adevice accordingto the invention.

i Plates-1 and 2* m'ade. ofandnsulatmg material such" as glass or plastic are. .inside coated 'witlfcondn'cting layers 3 and.4,srespectiv'ely," At-least one of theplates 1 and 2 and the conducting layer covering this plate are coatin gse" or 4 the layer thusprepared having a thickness ofabout 55 microns. 7 u

I I. ExarizpleS i 100 g. zincsuphos'elenide. phosphor (grain size up to '14' microns) r 2 00 g, filling substancefconsisting of titanium dioxide,

transparent or transliicent' inrord'er totr an s'mit light. The" V c1")ntlil'c'tirig la er-may preferably be' niadeiof-ainorphons arsenic-endear cadmium oxide; Itis" withinthe' pur and' Z with." f1 Onebfthe opaq at art. Th -"plates use: ma e-c a au t al im Wire's! 'Li h i em t fl" ,i la er"? -i ta n nse 'h h. ts w n iinorganic" 'di'e'lect rrc'fconstant. of :at lea angent'a) of'at most 0:2;v

oplat '1 is' e h i ye m b t e nti m x u 1 LL ph sph r o he nS- Yn a bind n iIig ftoithe' invention" a phqsi no r t egraame net to produce. an: electroluminescent} cell accordsuch asba i ,titanateof a high dielectric'cbnst'ant amt ojf'gifain size less thanS 'mi rp s. added mag htityfe fefredf'tof above. om thils'mixture and te a oma-y .ingmaterial and a Sammie ic it re aire'd afsofi. ell g agent a d t o galiic St fi sy ad 7 isef'b rought. up between the layers 3 and '4. The'thrcktnessbf this 11ylf fiS of about 25 200 microns, for-"example m ns- I hal d t9 b eva or ef c i I wAs a bi n ent gol qsl gn m ed efi a suspension isfmade and sprayed ori'oth hr: d ibut p th l t b t I 5 agents s s sl ose acsl o t e ke mayi use g.;hindin'g.material,' preferably. nitrocellulose f' V V are intimatelyjmiired with eachother and with 11 7 e "a solvent containing 150 g. softener .A layer. 8 made of. this suspension is sandwichedin between thejcjonductiv e layers. 3 and 4; the layer 8 -having a thickn Exani pZeiZ mo 30 micron V filling 'material ta mixture of iwa s a m V T ara'intimatelyniix d with' 'ea h n he rihen it) '1. ip v 1 100 g. zinc cadmium sulphide phosphor (grain size olfat 7 a a '50 sq y tmay-be butyla eta e o any other usua wl 1 2. An electrolu 0.34.1 m'icrsmj i t ia a ia sr le ni r eallal s e. n tel nae q aflir-j i s e q s rr i' zircon dioxide ns 'a flsetiaart taaats (seen s z as referred to in Examples 1 and 72 and a l ayer on thickj'ness ofabout 38 microns is spra'ed between the conducting c tings-sand 43 ,Exainple V7 100 3. zinc sulphide phosphor (grain 1 size up to 37 5 0 g. filling substance consisting 'of magnesium ifitanate: and 20%: barium zirsqn te ai e. mi I Q/ a are intimately'mixed with each other, then a suspension is"'prepardaefeferrd to in Examples 1 and 2; The

layer Smade of this suspension 'is of about microns in thickness; "1 i The electroluminescent cell made according to the invention can bereadily-"mass produced and the final unit 1 30 ise'asily handled and can be" simply connected 'also to the usual mains'gof'llQ or 220"volts for example! The light efii'ciency or the cell is fairly good as compared with 'known' electroluminescent cells and the lunienlvvatt lue is about 4- 5 or evenmor'e at the same timethec ell ,opcratingfsmoothly and free .of breakdowns.

While we have indicated the preferred embodiment of our invention of which we are now aware and have also indicatedonly one specific application for which' ourfinventionrnay be'cmployed, it will be apparent that our invention is by no means limited'to the exact form illust'r a t ed'andmanyvariationsmay benia'de. Instead of using phosphors of the Znsrt'y'pe anylother electrolun iinescent material may be used'as well as any other filling and bindingjmaterial without departing tr'om the s cope'of our invention as set forth in the appended claims. The

electrodes showngin the drawing 'forma'plates. In other embodiments of the invention the 1 electrodes may have any other form. f f 1""'We claim i 1. An electroluminescent" cell having a layer capable of ele ctroluminescence containinglagbinding m' rial and terial; whose dielectric constant is at least'20, ic' loss isfn'otmore than 0.2, whose Weight phosphor' pr esenti' and whosefparticIeQ size is not greater m ns pm s L nescentj'cell as claimed claim 1,

in which the finely granulated inorganic fillingsubst'ance onti uin 1 .0 -Ts ftene ed ed-Th suspension 7 thus prepared is'spra'yed onto one" Of .1 conducting;

V has a dielectri'cjco nstant of. about'SO, a dielectric loss 119mm t an 0.1 and particle size not eer tha '1 micron. s

3. An electroluininescentcell according 10 claim ,1, in

' whichthe ratio by weightot fillingsubstance to phosphor at' asta nt.3=4-

' electroluminescent celliaccordin g to "claim 1,

;. inwhich atleastpartof the filling substanco'is anoxide of anelement of group .lK/f'of the'periodic tahle.

5. An electroluminescentcell according to claiml 1 in which 'at .least'partlof. the filling' 'substance is t aruum Pai 6. An electroluminescent cell according to claim 1, in which at least part of the filling substance is zirconium oxide.

7. An electroluminescent cell according to claim 1, in which the filling substance is at least partly selected from the group consisting of titanates, zirconates, hafnates, and niobates.

8. An electroluminescent cell according to claim 6, in which at least part of the filling is selected from the group consisting of alkaline earth titanates, zirconates, hafnates, and niobates.

9. An electroluminescent cell according to claim 1, in which the binding substance is at least partly nitrocellulose.

10. An electroluminescent cell according to claim 1, in which the binding material is a mixture of nitrocellulose and an artificial resin produced by condensation and an artificial resin produced by polymerisation.

11. An electroluminescent cell according to claim 1, in which the binding material is a mixture of an artificial resin produced by condensation and an artificial resin produced by polymerization.

References Cited in the file of this patent UNITED STATES PATENTS 2,624,857 Mager Jan. 6, 1953 2,728,731 Butler et al Dec. 27, 1955 2,733,367 Gillson Jan. 31, 1956 2,765,419 Roberts Oct. 2, 1956 2,791,723 Nagy et al. May 7, 1957 FOREIGN PATENTS 204,854 Australia May 26, 1955 764,867 Great Britain Jan. 2, 1957 

1. AN ELECTROLUMINESCENT CELL HAVING A LAYER CAPABLE OF ELECTROLUMINESCENCE CONTAINING A BINDING MATERIAL AND A FILLER AND PLACED BETWEEN, AND IN INTIMATE CONTACT WITH TWO ELECTRODES, ONE AT LEAST OF WHICH IS SUBSTANTIALLY TRANSPARENT AND WHICH BOTH HAVE A RESISTANCE OF LESS THAN 1000 OHMS PER CM.2, SAID LAYER CONTAINING IN ADDITION A FINELY GRANULATED INORGANIC FILLING SUBSTANCE, ALSO IN THE BINDING MATERIAL, WHOSE DIELECTRIC CONSTANT IS AT LEAST 20, WHOSE DIELECTRIC LOSS IS NOT MORE THAN 0.2, WHOSE WEIGHT REPRESENTS BETWEEN 10% AND 200% OF THE WEIGHT OF THE PHOSPHOR PRESENT, AND WHOSE PARTICLE SIZE IS NOT GREATER THAN 5 MICRONS. 